Chemical aids for paper-making play an important role in the sustainable development of the paper-making industry, and therefore attract extensive attention. Chemical aids for paper-making can be classified into processing aids and functional aids. Strength aid is one of the functional aids. The strength parameters of paper comprise dry strength, wet strength and temporary wet strength, etc.
At present, frequently-used dry strength aids includes, for example, natural polymers such as cationic starch, CMC and guar gum, and synthetic polymers such as polyacrylamide (cationic, anionic and amphoteric), glyoxylated polyacrylamides (GPAMs), polyvinylamine, etc. Among the dry strength aids for paper, polyacrylamides (PAMs) are most widely used today. Categorized from their ionicity, the PAM type of paper dry strength aid is classified into anionic, cationic and amphoteric types. Since the early 1980's, amphoteric polyacrylamide polymers were developed by copolymerization of anionic vinyl monomers and cationic vinyl monomers, as well as acrylamide monomers (see JP1049839B). Di-aldehyde functionalized polyacrylamide, prepared from di-aldehyde and polyacrylamide, was developed first as a temporary wet strength resin (see U.S. Pat. No. 3,556,932A, U.S. Pat. No. 4,605,702A) back in the 1970's and 1980's. It was then developed as a dry strength resin used in combination with another wet strength resin, usually a polyamine-epichlorohydrin or polyamide-epichlorohydrin type of wet strength resin, as described in U.S. Pat. No. 5,674,362A. Glyoxalated polyacrylamide (GPAM), prepared from glyoxal and backbone polyacrylamide, is the most widely made paper dry strength aid in this category. Being used independently, anionic and amphoteric (WO0011046A1), as well as cationic (U.S. Pat. No. 7,641,766B2, U.S. Pat. No. 7,901,543B2) di-aldehyde functionalized polyacrylamide, mostly GPAMs, were developed to impart paper with enhanced dry strength, wet strength or drainage ability.
Dialdehyde-modified cationic, anionic and amphoteric acrylamide-containing polymer, particularly the glyoxylated DADMAC/acrylamide copolymers (GPAMs), is used as a dry strength and temporary wet strength aid in producing paper and paperboard. Such polymer strength aid is of great interest for paper and paperboard manufacturer since (1) it provides good temporary wet strength together with good dry strength, and (2) it also helps improve the paper machine runnability. Amphoteric acrylamide-containing polymer can provide the paper with good dry strength. Meanwhile, this type of strength aid possesses high content of active ingredients without any issue of shelf life. At present, both glyoxylated acrylamide copolymers and amphoteric polyacrylamides are widely-used strength aids. Further research and development on these two strength aids are rather intensive. Besides, the research on the combination use or the mixture of these two strength aids are carried out for the purpose of combining their respective advantages.
WO9806898A1 discloses a paper-making process, wherein cationic polymer selected from the group consisting of cationic starch and cationic wet strength resin, and amphoteric polyacrylamide-type polymer is added to the aqueous pulp slurry in order to increase the dry strength of paper, and wherein GPAM can be used as the cationic wet strength resin. Moreover, U.S. Pat. No. 6,294,645B1 discloses a dry-strength system for paper comprising PAE, amphoteric PAM and wet strength resin, wherein GPAM can be used as the cationic wet strength resin. JP2004011059A discloses a combination use of anionic polyacrylamide containing specific anionic monomer(s) and amphoteric polyacrylamide to strengthen the dry strength and drainage properties of finished paper. It is mentioned in the examples of this patent document that the aqueous solution of the anionic polyacrylamide was first adjusted to a pH value of 5.1-5.3, and then a 1% dilution of this aqueous solution of the anionic polyacrylamide was mixed with a 1% dilution of the aqueous solution of the amphoteric polyacrylamide prior to the addition into the slurry. JP2006138029A discloses a combination use of the anionic polyacrylamide and the amphoteric polyacrylamide to enhance the dry strength and drainage properties of finished paper. In the patent document, the following method is used: adjusting in advance the aqueous solution of the anionic polyacrylamide to a pH value of 6 or more, and then mixing a dilution of the aqueous solution of the anionic polyacrylamide and a dilution of the aqueous solution of the amphoteric polyacrylamide prior to the addition into the slurry. However, the above documents simply adjust the pH value of only the aqueous solution of the anionic polyacrylamide. Moreover, neither of the above documents discloses nor suggests adjusting the pH value of the mixed solution, not to mention the influence of adjusting the pH values of the mixed solution on the ash retention of the finished paper.
In recent years, ash retention of finished paper becomes one of the important parameters to evaluate the properties of finished paper as the massive utilization of the recycled paper pulp. Whether the ash retention of finished paper can be effectively improved while maintaining the strength effects for finished paper becomes one of the criterions to evaluate the comprehensive performance of the strength aids.
However, the existing strengthening agents, method of using those strengthening agent, and the related paper-making line are relatively stable. From the economical viewpoint, it is desired to, starting from the existing strengthening agents, modify its composition and the use method thereof to a minimal extent, and the strengthening agents would have a better effect of increasing the ash retention for finished paper.